For achieving uniform illumination of the image field and a high quality image, in an endoscope, the optical fibers must be arranged as far as possible concentrically about the inner tube. In the manufacture of endoscope optical systems, therefore, the optical fibers are distributed uniformly about the inner tube and are fastened to the inner tube, for example, by means of a thread. The inner tube is then inserted into the outer tube. Since no special-purpose device is available for centering the tubes, it may occur during the insertion of the inner tube into the outer tube, that the optical fibers are pushed out of their position on the periphery of the inner tube, so that the inner and the outer tubes become eccentrically located relative to one another and sections of the inner tube no longer extend parallel to the centre line of the outer tube. The resulting uneven distribution of the optical fibres at the distal end of the optical system produces defective, that is to say patchy, illumination of the image field and, because sections of the inner tube accommodating the optical components are not parallel to the outer tube, the image is of unsatisfactory quality. This centering problem is particularly acute in the manufacture of long and thin optical systems having a 0.degree. line of sight.
There is disclosed in DE-A-23 61 873 an apparatus for arranging a plurality of small-diameter optical fiber concentrically about a single large-diameter optical fiber. The large-diameter optical fiber is passed through a central guide sleeve of a wooden guide plate and the small-diameter optical fibers are passed through a plurality of guide sleeves disposed in the guide plate in one or more concentric circles about the central guide sleeve. Although this procedure allows of exact positioning of the optical fibers on the inner tube in the manufacture of endoscope optical systems, the problem of centering the unit so produced in the outer tube upon its insertion thereinto, is not avoided.